Make all assertions return true on success, so that it can be used inside PHP 7 assert()

[danizord]

/**
 * @param ThingInterface[] $things
 */
public function __construct(array $things)
{
    // no-op on production environment! :D
    assert(Assertion::allIsInstanceOf($things, ThingInterface::class));

    $this->things = $things;
}

@beberlei what do you think?

[gabrielsch]

good feature, and not bc break. 👍

[stof]

this looks good to me indeed

[rquadling]

Care to make a pull request? If it is non breaking, then will be easier to add.